Chick embryo ciliary ganglia and muscle cells, grown together in culture, form functional neuromuscular synapses. The proposed research projects involve the following: (1) The precise locations of synapses will be mapped by electrophysiological techniques. (2) The acetylcholine (ACh) sensitivity of individual innervated and noninnervated muscle fibers will be monitored in longterm experiments using electrophysiological techniques. (3) An E.M. study will be made of preformed "hot spots" of ACh sensitivity on the muscle fibers. (4) The effects of synaptically evoked muscle activity on muscle ACh sensitivity and on "hyperinnervation" will be studied. (5) Intracellular recordings will be made from cultured neurons to determine the source of their spontaneous activity.